To quantify the performance of experimental ocean current turbines (OCTs) during offshore testing, important performance metrics are presented along with a proposed sensor suite and the mathematical relationships that can be used to calculate them. A numerical simulation of an OCT and numeric models of the selected sensors are utilized to synthesize measurements which are used to calculate relative free stream flow velocity, electric power output, total system efficiency, shaft power, and rotor efficiency. By evaluating the calculated performance metrics, both with and without sensor limitations, the impact of the sensor limitations on the calculated performance metrics are evaluated. The impact of averaging times on the repeatability of performance calculations are also evaluated for several operating conditions to guide offshore testing requirements. It is found that for a 2 m significant wave height the selected sensor system increases the standard deviation of the calculated performance metrics for 1 min averages and an operating depth of 10 m by 5-20%. For a depth of 20 m the OCT performance is more consistent and the sensors increase the variability of the calculated performance metrics by between 30-50% for 1 min averages. For the same significant wave height and a depth of 10 m the standard deviation of the 1 min averaged sensor measured system and rotor efficiencies for are shown to range from 0.9 and 3.3% of their mean value. However this can be decreased to a 0.13 and 0.52% if the operating depth is decreased to 10 m and the averaging time is increased to 10 min.